1. Field of the Invention
The present invention relates generally to the field of continuous casting of metals, such as steel and, more particularly, is concerned with a discrete excitation coil incorporating multiple electrical conductor turns and independent hydraulic fluid flow paths for cooling the electrical conductor turns in order to produce a levitating and stabilizing force on the meniscus of the liquid metal sufficient to effectively seal the pouring tube outlet nozzle/mold inlet interface in a continuous casting machine.
2. Description of the Prior Art
Horizontal continuous casting of metals, such as steel, into the form of billets, slabs and the like are enjoying increasing interest in the continuous casting art. A preferred horizontal continuous casting method is to utilize a tundish which feeds molten metal via a pouring tube downwardly through a top inlet of a horizontal continuous casting mold. The pouring tube has an outlet nozzle which extends through the top inlet of the mold and into the pool of molten metal contained therein. The mold includes interconnected top, bottom and opposite side walls which contain the molten metal in the pool thereof below the top inlet and also typically define plural outlet ports through which solidifying strands of the metal in billet of slab form are independently withdrawn from the mold. A representative example of a horizontal continuous casting machine is disclosed in U.S. Pat. No 4,540,037 to Langner.
Typically, the horizontal continuous casting mold is oscillated in the horizontal direction in order to obtain a problem-free withdrawal of the strands and to realize a satisfactory surface quality of the casting. However, in order to minimize the amount of mass which must be oscillated and thus the complexity of the machine, it is advantageous to maintain the tundish and pouring tube stationary relative to the oscillating mold. It is therefore necessary to provide sufficient space at the interface between the pouring tube outlet nozzle and an annular rim formed in the top wall which defines the mold top inlet to allow such relative movement.
The amount of mold oscillation required is dependent on factors such as slab dimensions and casting temperature. For instance, oscillations of plus or minus three-eights inch are typical for slabs six by thirty inches in cross-section and composed of steel. This also necessitates that at the pouring tube outlet nozzle/mold inlet interface, a minimum mechanical clearance of at least three-eights inch must be provided to accommodate the oscillation.
Various approaches to sealing the inlet nozzle/mold inlet interface have been proposed, such as mechanical sliding seals, elastic or bellows-type seals, and high pressure air or inert gases. None of these methods have been successful. Specifically, the mechanical sliding seals were quickly found to develop leaks at the high temperatures and high duty cycles. The pressurized air-jet approach has been discarded due to the fact that it introduces air bubbles into the molten steel, deteriorating the tensile strength of the finished product.
One sealing approach that appears to have promise is to use a single-phase electromagnetic coil wound around the inlet nozzle, such as shown in the above-cited U.S. Pat. No. 4,540,037 to Langner. The coil has usually been made up of only a few turns, and typically has an excitation strength of 80,000 to 150,000 ampere-turns for pouring tube nozzle inlet diameters no greater than 5 inches. The large ampere-turn excitations are necessary because the height of the ferrostatic molten metal head extending from the tundish to the outlet nozzle of the pouring tube is often rather deep (such as 36 inches) and can be on the order of 10 psi. However, because of the high excitation ampere-turns and few turns, large line currents are required. These current levels are only available at the suboptimal power line frequencies of from about 50 to 60 Hertz.
Several major obstacles are presented by the use of a coil having relatively few turns. One obstacle is that the magnitude of the levitating or stabilizing force exerted on the molten metal in the pouring tube outlet nozzle region is too low to counteract typical ferrostatic heads in common use in large commercial casting systems. Another obstacle is that having only a few turns (1 to 3) means that the main lead current is on the same order of magnitude as the excitation ampere-turn requirement. This is an extremely inefficient arrangement in that the high currents (33-150 kilamps) required are not available at the optimum levitating frequencies (100-1000 Hertz) and although currents in this range have been supplied at power line frequencies (50 to 60 Hertz) it is impractical to supply such high currents given size and space limitations associated with continuous casting machines. Further, the main leads from the power source to the coil are much longer than the excitation coil length (for instance, 100 feet compared to 15 feet) and represent a more difficult cooling problem than the coil itself. Also, the few turns are of necessity relatively large in size and have a high "eddy factor"(eddy factor is the ratio of effective electrical conductor resistance at rated frequency to the conductor's electrical resistance to d.c. current). Additionally, there is a non-uniform magnetic field distribution about the periphery of the excitation coil. Yet another obstacle is that the discrete coil requires that an external mechanical frame support the coil and further that electrical insulation be provided between the coil and the mold. These requirements usually cause the distance between the bottom of the lowermost coil conductor and the melt meniscus to be unnecessarily large, reducing the electromagnetic pressure considerably.
A variant of the above approach of using a discrete coil with few turns is the use of a single-turn copper casting integral to the mold. While a significant advantage of this variant approach is that the current path is concentrated in an extremely robust, compact circuit close to the molten metal without the need for insulation, several major drawbacks are also present. One drawback is that, again, the supply current must be numerically equal to the excitation ampere-turns (e.g., 100,000 amperes). As mentioned earlier, this is an extremely high value and would require a complicated cooling scheme to keep the main lead conductors cool over their long length. Another drawback is that the current in the integral mold design would tend to concentrate at the top, outer surface rather than preferably at the bottom section near the molten metal. Still another drawback is that there exists at two points about the metal meniscus a magnetic null point either in the insulation space between inlet and outlet conductor leads or where the current divides in a one-half turn type arrangement.
In view of the aforementioned obstacles and shortcomings, a need still remains to find a practical way to use electromagnetic pressure as a seal around the pouring tube inlet nozzle. It appears certain that, in order for any alternative approach to have a chance for succeeding, it must address and eliminate the aforementioned obstacles and shortcomings of the prior discrete and integral coil arrangements.